Geophysical Monitoring of Hydrocarbon-Contaminated Soils

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Geophysical Monitoring of Hydrocarbon-Contaminated Soils Remediated with a Bioelectrochemical System Deqiang Mao, Lu Lu, Andre Revil, Yi Zuo, John Hinton, and Zhiyong Jason Ren Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b00535 • Publication Date (Web): 07 Jul 2016 Downloaded from http://pubs.acs.org on July 8, 2016

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Environmental Science & Technology

Geophysical Monitoring of Hydrocarbon-Contaminated Soils Remediated with a Bioelectrochemical System

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Deqiang Mao1, Lu Lu2, André Revil3*, Yi Zuo4, John Hinton1, and Zhiyong Jason Ren2

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Colorado School of Mines, Department of Geophysics, Golden, 80401, CO, USA

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Department of Civil, Environmental, and Architectural Engineering, University of Colorado Boulder, Boulder,

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Colorado 80309, United States

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ISTerre, CNRS, UMR CNRS 5275, Université de Savoie, 73376 cedex, Le Bourget du Lac, France

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Chevron Energy Technology Company, San Ramon, California 94583, United States

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* Corresponding Author: André Revil

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Phone: 33+ (0)479758715

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Emails: [email protected]

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1 ACS Paragon Plus Environment


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Abstract. Efficient non-invasive techniques are desired for monitoring the remediation process

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of contaminated soils. We applied the direct current resistivity technique to image conductivity

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changes in sandbox experiments where two sandy and clayey soils were initially contaminated

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with diesel hydrocarbon. The experiments were conducted over a 230 day period. The removal

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of hydrocarbon was enhanced by a bioelectrochemical system (BES) and the electrical potentials

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of the BES reactors were also monitored during the course of the experiment. We found that the

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variation in electrical conductivity shown in the tomograms correlate well with diesel removal

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from the sandy soil, but this is not the case with the clayey soil. The clayey soil is characterized

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by a larger specific surface area and therefore a larger surface conductivity. In sandy soil, the

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removal of the diesel and products from degradation leads to an increase in electrical

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conductivity during the first 69 days. This is expected since diesel is electrically insulating. For

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both soils, the activity of BES reactors is moderately imaged by the inverted conductivity

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tomogram of the reactor. An increase in current production by electrochemically active bacteria

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activity corresponds to an increase in conductivity of the reactor.

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TOC Art

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INTRODUCTION

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Hydrocarbon contamination due to accidental spills and tank leakages represents a threat

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to the environment1,2. Besides natural attenuation of the contaminants3,4, a number of engineering

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solutions have been applied to enhance the remediation process5,6. Recently, bioelectrochemical

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systems (BESs) were developed and tested for the remediation of hydrocarbon contaminated

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soils7-13. During the remediation process, the BES provides an environment for electrochemically

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active bacteria (EAB) to catalyze the oxidation of organic electron donors, e.g., hydrocarbon

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contamination, and deliver the electrons to the anode. Produced electrons are collected on the

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anode and transferred through an external circuit to the cathode, where O2 serves as the unlimited

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terminal electron acceptor9,14. Previous studies showed BESs could enhance hydrocarbon

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degradation by up to 241% in saturated soils12. In addition, no external energy is required by this

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process and in fact a small current is even produced that can be used in turn to power small

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electronic sensors for onsite monitoring.

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Even though BESs have demonstrated improved performance in hydrocarbon-

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contaminated soil remediation and provide current as a proxy for monitoring13, they still relies on

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traditional total petroleum hydrocarbon (TPH) measurements, which are intrusive and time

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consuming. The TPH measurement involves soil sampling, blending with volatile and toxic

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chemicals, plus multiple stages of extractions using vortex and sonication. Another issue is that

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the spatial coverage of TPH measurements is usually very limited. In order to provide more

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accurate, real time, and easy remediation monitoring, new methods that are fast and non-

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intrusive are urgently needed.

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Electrical resistivity survey is a non-invasive geophysical method consisting in imaging

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the electrical resistivity or electrical conductivity of porous media15. Electrical conductivity is 4 ACS Paragon Plus Environment

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sensitive to the salinity and temperature, water content, and cation exchange capacity (CEC) or

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specific surface area (SSp) of soils16. During a resistivity survey, an electrical current is injected

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through current electrodes, and at the same time, differences of electrical potentials are measured

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at voltage electrodes17-19. Resistivity survey has been applied to track the movement of saline

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plume20, 21 and contaminants22. For petroleum hydrocarbon contamination, a number of studies

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have been done to show the potential of resistivity survey monitoring23-25. Several laboratory

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experiments have been conducted on soil samples to understand the relation between electrical

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conductivity and hydrocarbon contamination by manually mixing different contents of

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hydrocarbon products with uncontaminated soil26-28. For non-wetting oils, e.g. diesel29, the

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measured conductivity is well correlated with hydrocarbon content, with a lower content

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corresponding to a higher conductivity. This is true for both fresh oil and biodegraded oil taken

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from field sites28.

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In the current study, diesel is chosen as the hydrocarbon contaminant and mixed with

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either sandy or clayey soil in two tanks for characterizing the BES reactor efficiency in

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accelerating hydrocarbon degradation. When non-conductive diesel is consumed by degradation

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and removed from soils, it is expected to increase the electrical conductivity of the material. A

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time lapse resistivity survey is designed to capture these changes during the tank experiment.

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Our goal is to establish the relationship between the diesel TPH content and measured electrical

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conductivity variations. At the same time, besides the monitoring of voltage generation from

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BES reactors, reactors were also evaluated in the resistivity survey in terms of conductivity

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variation.

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MATERIALS AND METHODS



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Tank Setup. The two tanks used for the analysis had a dimension of 40 cm × 5.5 cm ×

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20 cm (Figure 1). The BES reactor was placed on the left side of each tank. A column-type BES

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reactor was characterized by an outer diameter of 4.5 cm. A total of 32 stainless steel electrodes

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were fixed to the side wall of the tank. These electrodes were connected with a banana socket

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panel for resistivity surveys. Two sites at x = 5 cm and x = 39 cm which were 1 cm and 35 cm

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away from the reactor were used as soil sampling locations for TPH measurements. Three

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locations A, B and C, at x = 7 cm, 14 cm and 30 cm, were chosen as soil electrical conductivity

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measurements at the end of the experiment. For each of the three locations, soil samples at three

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different levels were taken.

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Two different types of soils, sandy and clayey soils, at saturated conditions were

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employed to test the effectiveness of the BES reactors. Two soils were prepared by mixing

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natural soils with pure silica (Type 7030 from Unimin Corporation) sand or pure clay (Pottery

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clay from Stone Leaf Pottery), respectively. Natural soil is needed for indigenous bacterial

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population. The natural soil characteristics were reported by Lu et al. [2014]13. After mixing,

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sandy soil (57.14% sand, 28.93% silt and 13.93% clay in dry weight) was characterized as sandy

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loam and clayey soil (33.12% sand, 35.46% silt and 31.42% clay in dry weight) was

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characterized as clay loam according to USDA soil texture classification. The porosity of sandy

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soil is 0.41, and 0.50 for clayey soil. SSp of sandy soil was measured at 1.68 m2/g, and 4.07 m2/g

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for clayey soil with 5-point BET method. The CEC values30 of sandy and clayey soils are

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calculated as 0.56 meq/100g and 1.35 meq/100g by assuming two elementary charges per nm2.

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Diesel was chosen to represent the hydrocarbon contaminants. Diesel was well mixed with soils

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before putting into tanks. Soils were saturated with artificial groundwater. Artificial groundwater


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refers to a solution of 4 mM Na+, 0.2 mM Ca2+, 0.1 mM Mg2+, 2.4 mM Cl-, 2 mM HCO3- , and

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0.1 mM SO42- in deionized water (conductivity ≈ 0.05 S/m at 22oC, pH ≈ 8.2)31.

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During the whole experiment and in order to minimize evaporation, both tanks were

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covered with plastic wraps on the surface excluding the area where the reactors were located.

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The hollow pipe of the reactor was left open for exposure with air, which provides the terminal

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electron acceptor in the form of O2. The water loss due to evaporation and leakage to the

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perforated BES reactors was approximately compensated by adding deionized (DI) water twice a

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week to maintain saturation. The saturated condition was observed from the moisture condition

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at the surface of each tank.

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Bioelectrochemical Reactor Operation. For both reactors, carbon cloth with activated

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carbon catalyst served as air cathode32-34. For the anode, a carbon felt anode was used. Cation

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Exchange Membrane (CEM) was employed as a separator between cathode and anode. Both

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reactors were wrapped around a perforated PVC pipe, and the height of the PVC pipe was 25 cm.

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Detailed information about how to construct reactors can be found in Lu et al. [2014]13.

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Continuous current generated from the remediation process was monitored and evaluated

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by an external circuit with a resistor during the whole experiment period. A 1000 Ω external

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resistor was utilized to start the experiment when a sustainable and relative large voltage

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(>100mV) was observed in a few days after the start of the experiment35, and at 162nd days it

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was replaced with a small resistor of 100 Ω to speed up the remediation process.

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Control experiment of natural biodegration was run in separate containers which had the

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dimension of 8 cm × 5 cm. There were no BES reactors and geophysical electrodes installed for

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the control experiments, and therefore TPH variation only reflects the natural biodegration. We



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took TPH measurements from the control experiment at the same time as the active tank

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experiment for comparison.

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Soil Sampling. Soil samples were taken throughout the experiment at two distances away

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(x = 5 cm and x = 39 cm) from the reactor (Figure 1) for TPH measurements36. At each location,

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5 g of soil from the top, middle and bottom of the tank were sampled and then mixed for TPH

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measurements. Seven TPH measurements at day 0, 8, 40, 60, 120, 150 and 200 were conducted

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at each location during the whole experiment. The initial TPH values for sandy and clayey soil

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were 6960 mg/kg and 6495 mg/kg, respectively.

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At the end of the experiment, soil samples were taken at three different locations A, B,

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and C (Figure 1) for electrical conductivity measurement, at each location, lower (z ≈ 1.5 cm~4

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cm), middle (z ≈ 6.5 cm~9 cm) and upper (z ≈ 11.5 cm~14 cm) levels soil samples were taken.

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At each location, the average conductivities from all three levels were compared with results

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from the resistivity tomograms. Soils conductivity was measured in a 1:5 (w/v) soil: deionized

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(DI) water mixture37. With this method, the initial electrical conductivities for sandy and clayey

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soils were 558 µS/m and 1109 µS/m, respectively.

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Soil Column Conductivity Measurements. Measured conductivity tomograms are

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affected by both surface conductivity and pore fluid conductivity, and these two contributions

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cannot be separated in a resistivity survey. Therefore besides the tank experiment, soil column

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experiments were setup to separate the influence of surface conductivity and diesel TPH content

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for the two different types of soils. A four-electrode configuration (two current electrodes and

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two voltage electrodes) was adopted. The soil column had a diameter of 3 cm and a length of 11

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cm. Two stainless steel plates were used as current electrodes at both ends of the column, and

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Ag-AgCl potential electrodes were inserted in the middle of column with a spacing of 3 cm. The


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soil samples were saturated in a vacuum chamber. All the measurements were performed with

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ZEL-SIP04-V02 impedance meter38. The real part of the complex conductivity at 1Hz was

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chosen to represent electrical conductivity during a resistivity survey for analysis.

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According to Waxman and Smits39, electrical conductivity ߪ (in S/m) of the material at low frequency can be represented as ଵ

ߪ = ߪ௪ + ߪ௦ , ி

(1)

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where F (dimensionless) is the formation factor related to the connected porosity of the soil by a

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power law relationship known as Archie’s law, ߪ௪ is the pore water conductivity, and ߪ௦ is the

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surface conductivity associated to electrical conduction in the electrical double layer coating the

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surface of the grains30. This surface conductivity is controlled by the SSp or the CEC of the clays.

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Six electrolytes were obtained by mixing DI water and pure NaCl at the following conductivity

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(at 298 K) ߪ௪ =0.002 S/m, 0.02 S/m, 0.08 S/m, 0.2 S/m, 0.8 S/m and 2.0 S/m.

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For the TPH values in the soils, four different ratios were tried, TPH=0.0 g/kg, 7.5 g/kg,

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13 g/kg ad 32 g/kg, and 0.0 g/kg means no diesel was added to the soil. Diesel was added to soil

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first and artificial groundwater was then added. With the added diesel, it took a longer time to

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expel air from the mixture (approximately 3 hours in this study) compared with only water as

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pore fluid (0.6 hour) in a vacuum chamber. Potential loss of diesel due to evaporation was not

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accounted. In such a short measurement period, there was also no consideration of the natural

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degradation of diesel contaminants on the electrical conductivity.

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Resistance Data Collection and Inversion. Resistivity surveys were conducted eight

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times throughout the experiment (Figure 2), at day 12, 26, 42, 69, 105, 135, 187 and 230, with a

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two week interval at the beginning of the test and a 1.5 month interval at the end. For each

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survey, 480 resistance measurements were taken to cover the whole tank with a multichannel



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instrument ABEM LS and survey duration was 15 min. For the 480 measurements, current

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electrode pairs AB were chosen as 14 horizontal pairs and 3 vertical pairs, i.e. AB=15 and 21, 2

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and 7, 21 and 27, and 15 and 2. For each current injection pair AB, potential electrodes pairs

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were chosen with skip one scheme18. For example, for AB=2 and 15, potentials electrode pairs

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MN=3 and 5, 4 and 6, 5 and 7, and so on.

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All the 480 measurements were used for inversion. For each tank, the resistances from all

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eight surveys were inverted together with the active time constraint (ATC) technique40, 41. This

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technique improves the time-lapse imaging especially when the survey data is corrupted with

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strong noise. Due to the sensitivity difference of a resistivity survey with higher sensitivity close

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to the electrode arrays19, we presented the results in terms of conductivity ratio, which better

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shows the variations. The result from the first survey (at day 12) was treated as a reference. The

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conductivity ratio between the other seven surveys and the first one was presented for discussion.

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In this way, a conductivity ratio greater than 1.0 indicates an increase of electrical conductivity.

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The inversion was performed with a three dimensional finite element code built in the

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Comsol and Matlab environments41. Because of the smaller tank thickness in the y direction, we

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assumed no conductivity variation in the y direction. Therefore the inversion results were

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presented as two dimensional cross-tomograms.

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RESULTS AND DISCUSSION

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Voltage Generation. Figure 2 shows the voltage profile for the two tanks. Stable voltage

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was generated in both tanks. At the beginning the electrical voltage has a rising limb due to

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acclimation of indigenous microbial electrochemical community, and then the voltage reaches a

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stable value. A longer delay period is observed in the clayey soil to reach the stable value due to 10 ACS Paragon Plus Environment

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slower mass transfer rate compared with the sandy soil from the restrict passage of solutes in

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clayey soils42. For both tanks, a stable voltage is reached at around 160 mV.

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After 162-day experiment, a 100 Ω resistor was used to replace the 1000 Ω to allow

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faster electron flow35. Both reactors show immediate voltage fluctuation in response to the

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resistor change and then the voltage decreased gradually with time. The immediate surge is

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caused by the reestablishment of the microbial activity with the smaller resistor.

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TPH Removal. The results from both tank and control experiments are shown in Figure

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2 (Y axis on the right). Corresponding to the initial increase of voltage, there is an increase of

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TPH close to the reactor at x = 5 cm for both tanks. This is due to the establishment of EAB

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activity with high adsorption of carbon felt anode attracting more diesel contaminants towards

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the reactor.

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Generally, TPH has larger removal rate closer to the reactor (x = 5 cm) and relatively

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lower rate further away (x = 39 cm) 13. However, the difference between these two distances is

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very small, especially for sandy soil. This proves the relatively large radius of influence under

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sandy soil condition. A higher TPH removal rate close to the reactor compared to TPH further

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away creates a diesel concentration gradient. This concentration gradient could drive diesel

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contaminant to diffuse towards the reactors. Another driving force is the hydraulic gradient

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created by the perforated BES reactors. The continuous leaking of liquid into the reactors and

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compensation from the adding DI water on the surface also drives the movement of diesel

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contaminant. Since sandy soil has a larger hydraulic conductivity which permits a fast movement,

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the difference between TPH measurements at the two distances is smaller compared to the clayey

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soil.



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Sandy soil shows a higher removal rate than clayey soil based on the general trend. Both

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BES experiments have higher TPH removal rate than control experiments, but less evident for

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clayey soil. Clay soil at x = 39 cm is almost close to the control test. First two TPH

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measurements before day 8 are indistinguishable with control experiment. From the

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measurement at day 40, we start to see the difference. At day 60, the difference becomes clearer.

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In the control experiment, for sandy soil, TPH decrease to 5200 mg/kg which corresponds to 25%

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removal rate, and 5550 mg/kg for clayey soil with removal rate 15%. With BES reactor, sandy

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soil has remove 41% and 35% TPH at x = 5 cm and at x = 39 cm respectively which are 64% and

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40% higher than natural biodegration. As expected, even with BES reactor, the TPH removal

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rate of clayey soil is not as good as sandy soil. At day 60, it only has 23% and 17% TPH

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removed at the two distances which is slightly higher than the control test value 15%, only 53%

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and 13% increase from the control experiments. At the end of the experiments, sandy soil has

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TPH values of 4100 mg/kg and 4210 mg/kg at x = 5 cm and 39 cm, which correspond to 41%

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and 39% removal rates based on the initial TPH content. For the control test, it has a removal

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rate 34%. For clayey soil, they are 4061 mg/kg at x = 5 cm and 4861 mg/kg at 39 cm which

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correspond to 35% and 25% removal rates. As for the control test, it only removes 22% of the

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TPH. From both tests, we can see that natural biodegration from the indigenous microorganisms

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plays an important role during the TPH removal. Therefore, when analyzing the resistivity

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change from the experiments, the change reflects the contribution from both BES and natural

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biodegration.

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Conductivity of Soil Column Measurements. The measured conductivity at 1Hz with

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different salinities is shown in Figure3a obtaining the influence of surface conductivity. The

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measured data was fitted with the relationship in Eq. (1). With this fitting, the obtained formation


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factors are 4.4 and 2.9 for sandy and clayey soil respectively. Clayey soil indicates a larger

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surface conductivity ߪ௦ = 0.051 S/m and sandy soil has a relatively smaller surface conductivity

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ߪ௦ = 0.022 S/m. With both fluid conductivity and surface conductivity affect the surveyed

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conductivity values, a larger surface conductivity corresponds to the higher clay content and also

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indicates the smaller influence from the change of pore water conductivity, which makes

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conductivity variation less evident when removing diesel from the soil.

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In Figure3b, ߪ଴ represents the electrical conductivity of soil sample without diesel

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contaminant (0.0 g/kg). Both sandy and clayey soils have a decreasing conductivity trend with an

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increasing TPH content as indicated in Figure3b. However, clayey soil shows a smaller decrease

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trend with only a 5% drop at 32 g/kg TPH due to a larger surface conductivity. Conductivity of

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sandy soil drops by 6% even at 7.2 g/kg TPH, and 22% drop when TPH reaches 32 g/kg.

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Conductivity Ratio Tomograms. Figure 4 is the conductivity ratio tomograms for both

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tanks where the sub titles show the survey day after the start of the experiment (12 being the

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reference). For the analysis of the soil conductivity variation, we exclude the reactor region

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marked by the dashed rectangles in black, and the conductivity variation of the reactors will be

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evaluated separately. Since we didn’t have a parallel tank experiment to monitor the change of

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conductivity with diesel removal under natural biodegration, the observed electrical conductivity

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variation represents the contribution from both natural and enhanced biodegradations from BES

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reactors.

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Compared to clayey soil results in Figure 4b, there is a distinct variation trend for

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saturated sandy soil (Figure 4a), which indicates the removal of TPH from the tanks. In Figure 4a,

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in the region closer to the surface, we observed a continuous drop of conductivity indicated by

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the blue color. There are two reasons for this drop. There is a buoyance effect from the diesel



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contaminants. Diesel has a density 0.832 kg/L and therefore has a tendency to move up above the

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pore water phase. A few weeks after the start of the experiment, there was a visual existence of

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diesel spots on the surface of the sandy tank. Another reason is due to added DI water on the

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surface to keep the soil saturated. Even though the tank is covered with plastic wrap to minimize

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evaporation, the tank is not completely sealed and water is still lost from the surface under

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Colorado’s dry climate. DI water is added to the surface twice a week. The added DI water only

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has a conductivity of 10-4 S/m and therefore this low conductive pore water reduces the

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conductivity of the sandy soil. The added DI water is believed to be the main reason for the

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observed decrease of conductivity on the top layer of the soil.

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From the tomograms, in the region below the surface, there is an increase of electrical

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conductivity at first, which is then followed by a decrease trend. However, the variation is not

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uniformly distributed inside the tanks. This is potentially from the uneven distribution of diesel

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contaminants inside the tank. In some regions, there is an increase of conductivity by 30% (Day

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69 in Figure4a) even with only 3 g/kg TPH removal, from approximately 7g/kg to 4g/kg (see

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Figure2a the second and fourth TPH measurements). This is a much bigger variation than the

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conductivity measurements shown in Figure 3b. There may exist some higher diesel content

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spots in the tank, and the organic acids and bicarbonate accumulation from the degradation also

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contribute to the increase of conductivity which is not accounted in the soil column

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measurements. As discussed by Allen et al. [2007]43, an increase of culturable hydrocarbon-

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degrading microbes is often observed in petroleum-contaminated sites. The presence of certain

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bacterial phyla corresponds to an increase of conductivity. Microbial activity increase from BES

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and natural biodegration can alter the subsurface electrolytic and interfacial properties of porous

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media. This alternation can influence the electrical conductivity25, 44.


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For the clayey soil, there was less variation in the conductivity ratio tomograms (Figure

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4b). We also observed some diesel spots on the surface of the tank after a few weeks of operation.

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However, the conductivity decrease trend close to the surface is not as clear as the sandy tank.

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The DI water moves slower in the clayey soil due to the low hydraulic conductivity, and

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therefore does not lead to a significant decrease to conductivity as indicated in the sandy soil (see

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Figure 4a).

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There is a significant increase of conductivity close to the reactor at 10 cm at the late

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stage of the experiment. This happens after the change of smaller resistor (162 day) with a

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concomitant reactor activity speedup. This increase is related to more consumption of diesel

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contaminants close to the reactor.

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Average Conductivity and TPH Variations. Because of the heavy soil sampling

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activity at the two locations x = 5 cm and 39 cm (Figure1), the soil was highly disturbed in these

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two regions. Therefore, we only consider the area inside the white dashed rectangle for

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calculation of average conductivity variation (the area is indicated by white dashed rectangles in

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the Figure 4a and Figure 4b Day 26). Solid lines in Figure 5a and 5b show the average

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conductivity ratio variation for both tanks for the area inside the white dashed rectangles

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indicated in Figure 4 (The last survey at 230 days is not used since it is beyond the last TPH

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measurement). Figure 5a and 5b also have the average TPH ratio from the two locations (x = 5

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cm and 39 cm) in dashed lines. The average TPH is calculated by taking the average of the TPH

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measurements at the two locations to represent the TPH variation inside the tank. Since the

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conductivity survey and TPH measurements are not taken at the same time, the average TPH

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values at the same time of the conductivity survey are linearly interpolated. All TPH ratios are

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referred to the interpolated TPH value at 12th day when the first conductivity survey taken.



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As shown by the average conductivity, the conductivity increases first and then

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decreases. The average conductivity ratio increases by about 12% at the fourth survey 69 days

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after the start of the experiment and then slowly decreases by 5% at the seventh survey (187 days)

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compared to the initial state. For the average TPH ratio result of sandy soil, there is a significant

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drop for the first three results in Figure5a, and then followed by a slight increase and decrease

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variation trend. The TPH decrease trend of the first three points corresponds to an increase of

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average conductivity. This is consistent with previous research conclusions that a lower TPH will

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reflect a higher conductivity value26-29.

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After the first three data points in Figure 5a, we could not find this relation anymore

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because there is another reason responsible for the observed variation of average electrical

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conductivity. Indeed, there are two processes occurring at the same time during remediation

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causing the change of electrical conductivity: (i) the removal of diesel contaminants and (ii) the

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leakage of the pore fluid into the reactor. Nonconductive diesel is removed from soil through

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biodegration and leads to organic acids and bicarbonate accumulation. This leads to the increase

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of soil conductivity. At the same time, because the whole reactor is wrapped around a perforated

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PVC pipe to maximize the cathode contact with air, pore fluid leaks through the perforated pipe

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and creates a hydraulic gradient towards the reactor which is enhanced by the added DI water on

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the surface. As a result of this head gradient, the removal of fluid and icons loss in the fluid from

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the tank decrease conductivity of the soils. The saturation maintained by adding low conductive

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DI water on the surface of the tanks could not compensate the decrease of conductivity.

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The effect of leakage enhances both water flow and solute transport in this experiment.

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Compared to solute transport under natural hydraulic gradient, e.g. 1%, which only moves a few

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meters per year45, BES reactor itself creates a favorite situation for the transport of contamination.


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In the tank experiment, if we assume the water table is approximately on the soil surface, and it

354

decrease to about 5 cm below the surface where it starts the perforated part of the PVC pipe. The

355

average hydraulic gradient can be calculated

356

as infiltration as what naturally happens from predipitation23, and the sandy soil example shows

357

us that it can help the movement of solute, and therefore it will help the remediation process46.

ଷହିସ.ହ ହ

≈14%. The act of adding water can be treated

358

For the sandy soil, the conductivity trend confirms that higher TPH removal rate is more

359

prevailing than the leakage of fluid at early period. At later period, the TPH removal rate slows

360

down as shown by the TPH variation. The decrease of conductivity from leakage is more

361

dominant which causes the decrease of surveyed conductivity. The smaller surface conductivity

362

for the sandy soil (ߪ௦ =0.022S/m) supports more influence of pore water in this study.

363

For the clayey soil in Figure 5b, we only observed a decrease trend in conductivity,

364

reaching 10% at 187th days with respect to the reference. The TPH ratio also continues dropping

365

throughout the experiment. The removal of diesel does not lead to an increase of conductivity at

366

the beginning of experiment compared to the results in Figure 5a for the sandy soil. As

367

confirmed by soil column results, this is due to the large surface conductivity of the clayey soil

368

(ߪ௦ = 0.051 S/m). Leakage of fluid is more important for conductivity variation throughout the

369

whole experiment. The removal of fluid decreases fluid conductivity, and therefore what is

370

shown in Figure 4a is a steady decrease.

371

Lateral Conductivity Variations. At the end of the experiment, we took soil samples

372

from the three locations A, B, and C (Figure 1) to evaluate the spatial variation away from

373

reactors. The conductivity ratio from the last survey at these three locations was also extracted

374

from the inverted tomograms. The extracted areas are indicated by three solid white rectangles in

375

Figure 4. Figure 5c is the comparison between sampled conductivity ratios and inverted values at



Page 18 of 31

376

8th survey from the two tanks. In Figure 5c, from both soil samples, there is a decrease trend

377

away from the reactor at locations A to C, indicating the loss of ions during the leakage process

378

throughout the duration of the experiment and its influence at the later period of the experiment.

379

This is supported by the tomograms of sandy soil figure4a. The increase of conductivity closer to

380

the reactor (indicated by red color) is always higher than the value further away, especially in the

381

last tomogram at Day 230.

382

The cross plot between inversion results and measured conductivity values shows a linear

383

relationship which means the geophysical survey can capture the conductivity variation in the

384

lateral direction. The relationship is not 1:1. The first reason is because of the uneven diesel

385

distribution in the tank initially, and there is only one initial value taken from each tank. When

386

taking the ratio with respect to the initial condition for all the samples, it introduces some

387

uncertainty. For the numerical inversion results, the ratio is taken with respect to the conductivity

388

in A, B, and C locations from the first survey at 12th day respectively. Another reason is the way

389

we measured electrical conductivity by drying the soil first and then mixing with DI water with

390

1:5 ratio37. With this method, the measured conductivity from soil samples can only be used

391

relatively compared to the in situ measurements from resistivity surveys.

392

Relationship between Electrical Conductivity and Voltage in BES Reactors. There is

393

no soil existing in the region of reactors with a hollow pipe design. Therefore, the reactor

394

conductivity variations reflect the activity on the anode of the reactor itself rather than bulk

395

conductivity of the soil. We analyzed conductivity variation of both reactors separately. During

396

the experiment, we also observed a conductivity variation for the reactors from the inversion

397

results. As shown in Figure 4a, the conductivity ratio for the reactor was closer to 1.0 first, and

398

then became smaller to 0.4 in blue color for the 6th survey Day 135. It then kept at the same


Page 19 of 31


399

range. If we see the voltage curve in Figure 2a, the first survey on the 12th day was taken when

400

its magnitude reaches the peak, and is then followed by a gradual decrease. The conductivity

401

ratio and voltage ratio from the reactors are plotted in Figure 5d. For the voltage ratio, the

402

magnitudes 316 mV and 83 mV for sandy and clayey soils at day 12 was utilized as a reference.

403

As indicated by Figure 5d, it follows a moderate linear trend with a scattered pattern for

404

the clayey soil. The linear trend supports that a higher voltage corresponds to a more conductive

405

reactor in the inverted tomograms. The mechanism behind this linear trend can be related to the

406

activity of EAB. When the voltage of the BES reactor is high, that means the EAB activity is

407

high. More diesel contaminants are consumed, and therefore more electrons are produced. This

408

leads to the creation of a conductive anomaly during the resistivity survey.

409

We have demonstrated a useful non-invasive monitoring technique for diesel removal in

410

a tank experiment using resistivity survey. An increase of conductivity corresponds to a decrease

411

of TPH content in sandy soil at the early period of the experiment, but not for clay soil due to the

412

high surface conductivity, and these tank results are further supported by separate soil column

413

measurements with different TPH contents. The leakage of the fluid through the perforated BES

414

reactors causes a decrease of conductivity temporally indicated by the inverted conductivity

415

tomograms and spatially supported by the soil samples at the end of the experiment, and fluid

416

leakage also acts as a driving force for the movement of diesel contaminant towards the reactors.

417

Besides the traditional voltage monitoring of BES reactors, the activity of the BES reactors is

418

characterized by the magnitude of the conductivity in the inverted tomograms as well. A more

419

active BES reactor corresponds to an increase of conductivity in the conductivity tomograms.

420

Even though these results are only from a lab scale tank experiment, the encouraging results

421

support the usage of geophysical survey during contamination remediation monitoring. Further



Page 20 of 31

422

large scale field tests are still needed to evaluate this technique under more complicated

423

environment to understand more influence parameters of electrical conductivity.

424 425 426

ACKNOWLEDGEMENTS

427

This work is funded by the Office of Science (BER), US. Department of Energy (awards DE-

428

SC0007118) and Chevron Energy Technology Company (grants CW852844 and K26625). We

429

also thank the specific surface area measurement from Dr. Ryan Richards’ lab of Colorado

430

School of Mines.

431


Page 21 of 31


432

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433

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biodegraded oil in porous media. Geophys. J. Int. 2014, 196, (2), 804-817.

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29. Schwartz, N.; Huisman, J. A.; Furman, A., The effect of NAPL on the electrical properties of

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unsaturated porous media. Geophys. J. Int. 2012, 188, (3), 1007-1011.

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30. Revil, A., Spectral induced polarization of shaly sands: Influence of the electrical double

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layer. Water Resour. Res. 2012, 48, (2), W02517.

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31. Hort, R. D.; Revil, A.; Munakata-Marr, J.; Mao, D., Evaluating the potential for quantitative

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electrical conductivity. Water Resour. Res. 2015, 51, (7), 5239-5259.

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32. Yazdi, H.; Alzate-Gaviria, L.; Ren, Z. J., Pluggable microbial fuel cell stacks for septic

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wastewater treatment and electricity production. Bioresource Technol. 2015, 180, 258-263.

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33. Haeger, A.; Forrestal, C.; Xu, P.; Ren, Z. J., High performance spiral wound microbial fuel

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cell with hydraulic characterization. Bioresource Technol. 2014, 174, 287-293.

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41. Mao, D.; Revil, A.; Hort, R. D.; Munakata-Marr, J.; Atekwana, E. A.; Kulessa, B.,

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Resistivity and self-potential tomography applied to groundwater remediation and contaminant

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plumes: Sandbox and field experiments. J. Hydrol. 2015, 530, 1-14.

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membrane barriers. Eng. Geol. 2003, 70, (3–4), 235-248.

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43. Allen, J. P.; Atekwana, E. A.; Atekwana, E. A.; Duris, J. W.; Werkema, D. D.; Rossbach, S.,

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44. Williams, K. H.; Ntarlagiannis, D.; Slater, L. D.; Dohnalkova, A.; Hubbard, S. S.; Banfield, J.

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45. Panday, S.; Wu, Y. S.; Huyakorn, P. S.; Springer, E. P., A three-dimensional multiphase flow

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simulation examples. J. Contam. Hydrol. 1994, 16, (2), 131-156.

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141, 62-70.

550


Page 27 of 31


Figures

551

552

Reactor

Electrodes

TPH locations

EC location A

EC location B

EC location C

12 7

18

1 13

32 26

20

Z (cm)

8

27 3

35

21

) cm Y(

15 3 0

5

25 15

X (cm)

553

554 555 556 557 558

Figure 1. Tank configuration used for the experiment. Current from the BES reactor was monitored by a data acquisition system. A total of 32 resistance electrodes (black cubes) were attached on the side wall of the tank. Two sites (x = 5 cm and 39 cm, light blue) were used as TPH sampling locations during the whole experiment. Finally, 9 soil samples were taken from 3 locations A, B, and C at the end of experiments for electrical conductivity (EC) measurements.

559



Page 28 of 31

560

561

a) Sandy soil sandbox experiment TPH-5 cm TPH-39 cm Control

300

8000

200 6000 100

0

0

50

100

150

200

TPH (mg/kg)

Voltage(mV)

400

4000

Time(day)

b) Clayey soil sandbox experiment

8000

TPH-5 cm TPH-39 cm

300

Control

200 6000 100

0

0

50

100

150

200

TPH (mg/kg)

Voltage(mV)

400

4000

562

Time(day)

563

Figure 2. Time-series of the voltage generation of BES reactors and TPH degradation during the experiment. a) Sandy soil. b) Clayey soil. Eight resistivity surveys were conducted at day 12, 26, 42, 69, 105, 135, 187 and 230. They are indicated the inverted filled triangles. TPH variation measured for each tank and the results are referred to the Y axis on the right-side of the plot. Seven TPH measurements were taken at day 0, 8, 40, 60, 120, 150 and 200. Control experiments of the two soils were prepared in different containers without BES reactors. It represents the natural biodegration. The vertical solid lines at day 162 indicate the change of resistor from 1000Ω to 100Ω.

564 565 566 567 568 569 570


Page 29 of 31


a) Conductivity data at saturation Conductivity of the soil (S/m)

0.60 0.40 0.20

Sandy soil Clayey soil

0.05

0.01 -3 10

Conductivity ratio (unitless)

574 575 576 577 578 579 580 581

100

Clayey soil 0.95

0.90

0.85

Sandy soil 0.80 0

573

10-1

b) Conductivity ratio vesus TPH

1.00

571 572

10-2

Pore water conductivity (S/m)

10

20

30

TPH (g/kg)

Figure 3: Petrophysical measurements. a) The solid dots show the electrical conductivity at 1Hz with different salinities for sandy and clayey soils. The data were fitted by a linear conductivity equation given by Eq. (1) of the main text. The fit of the data with this equation is materialized by the solid lines. Clayey soil has a bigger surface conductivity (ߪ௦ = 0.051 S m-1) than sandy soil (ߪ௦ = 0.022 S m-1). A bigger surface conductity will lower the influence of pore fluid conductivity created by diesel contaminants removal in this experiment. b) Electrical conductivity ratio σ / ߪ଴ decreases with increasing TPH. ߪ଴ denotes the soil conductivity with no diesel added in the soil sample.Sandy soil has a larger decrease rate than clayey soil due to a smaller surface conductivity as indicated the in Figure 3a.

582 29 ACS Paragon Plus Environment


Page 30 of 31

583

a) Sandy soil sandbox tomograms Day 42

R E A 10 C T 5 O R

Z (cm)

Day 69

R E A C T O R

15

R E A C T O R

Day 105

Z (m)

Day 26

R E A C T O R

0

Day 187

Z (cm)

0

Day 230

Z (m)

R E A C T O R

R E A C T O R

Conductivity ratio Z (m)

Day 135 R E A 10 C T 5 O R 15

0

0

0.3

0.2

0.5

0.7

0.9

1.1

1.2

1.4

x

5

10

15

20

X (cm)

25

30

35

b) Clayey soil sandbox tomograms Day 42

Z (cm)

Day 69

R E A C T O R

R E A C T O R

Day 105

Z (m)

Day 26 R E A 10 C T 5 O R 15

R E A C T O R

0

Z (cm)

0

584

585 586 587 588 589 590 591 592 593

Day 230 R E A C T O R

Conductivity ratio Z (m)

Day 187 R E A C T O R

Z (m)

Day 135 R E A 10 C T 5 O R 15

0

0

0.2

0.3

0.5

0.7

0.9

1.1

1.2

1.4

x

5

10

15

20

X (cm)

25

30

35

Figure 4. Electrical conductivity ratio tomograms with respect to the first tomogram of electrical conductivity taken at day 12. a) Sandy soil. b) Clayey soil. Blue color (1.0) indicates the removal of diesel contaminants. Title of each subfigure denotes the measurement day after the start of the experiment. The dashed rectangle (on the left side) indicates the location of the reactor. The dashed rectangles in white in the first figure show the area used to determine the average electrical conductivity used in Figure 5a and 5b. The solid rectangles (last plot) are three soil sample locations A, B and C at the end of the test. The solid black lines are the locations of surface and borehole electrode shown in Figure 1.

594


Page 31 of 31


595

596

c) Measured vesus inverted conductivity ratios

a) Conductivity and TPH ratio variations: sandy soil 1.2

1.05

Conductivity

0.8

TPH

0.95

TPH ratio

Conductivity ratio

0.9

0.7

0.85 10

40

70

100

130

160

Meausurements ratio

1.15

0.6 190

A

Sand

B C

0.9

A

Clay

B C

0.6

0.3 0.6

0.9

1.2

1.5

Inversion ratio

Time (day)

b) Conductivity and TPH ratio variations: clayey soil

d) Comparison between conductivity and voltage ratios

1.15

2.5

0.8

TPH 0.95

598 599 600 601 602 603 604 605 606 607 608

Sand Clay

1.5

1

0.5

0.85 10

597

0.7

Conductivity

Voltage ratio

2 1.05

TPH ratio

Conductivity ratio

0.9

40

70

100

130

160

0.6 190

0

0

0.5

Time (day)

1

1.5

2

2.5

Conductivity ratio

Figure 5. Electrical conductivity and TPH ratio variations for a) sandy soil and b) clayey soil. Electrical conductivity ratio is calculated from tomograms in the dashed white rectangle area in Figure 4 (except last survey at 230th day), and TPH ratio variation is averaged from the two TPH sampling locations at x = 5 cm and x = 39 cm. For sandy soil, an increase of conductivity corresponds to a decrease of TPH for the first three data points. c) Comparison of the measured conductivity ratio of the sandy and clayey soil samples (measured from tanks) versus the inverted conductivity ratio at the same locations A, B, and C from the inverted tomograms. This linear relation shows that the laboratory measurements are consistent with the tomogram data. d) Linear relationship between the inverted conductivity ratio and the measured voltage ratio for sandy soil (red dots) and clayey soil (green dots) for BES reactors indicated by the dashed rectangles in black in Figure 4.

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Geophysical Monitoring of Hydrocarbon-Contaminated Soils

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